PALAEONTOLOGY OF NEW-YORK. 
74 
Among a large number, the gradations to S. bifurcata, where the striae are 
regularly bifurcating and of nearly equal size, are almost imperceptible. 
It is apparently as the individuals become larger that there is an increase 
in the gibbosity of the dorsal valve,, while at the same time the striae 
become somewhat regularly alternating in size, and in some specimens 
nearly equal, but retaining a sharpness of outline not observed in 
specimens from the limestone. 
It was to those specimens with sharp equal strise and gibbous dorsal valves that 
I originally gave the name Strophomena arctostriata (Report of Fourth District). 
Specimens of this kind approach, both in form and expression, the figures of 
Streptorhynchus crenistria , illustrated by Mr. Davidson (Monograph of British 
Carboniferous Brachiopoda), where figs. 2 & 8, Plate xxvii, maybe compared with 
figs. 8, 11 & 14 of Plate x of this volume. The British Carboniferous specimens 
are larger than ours of the Chemung group. 
The gradations of form and character are pretty well shown on Plate x. Figs. 
1-6 are of the smaller forms, having the prevailing-surface-marking characteristic 
of S. pectinacea . The ventral valve fig. 2, and the cast of ventral and dorsal valves 
respectively figs. 3 & 5, are more like S. bifurcata. 
In figs. 7 & 8 we have gibbous dorsal valves, somewhat depressed towards the 
front, and with the striae subequal or alternating; and fig. 10 is the ventral valve 
of similar character. 
In the figs. 11 - 17, we have specimens with somewhat unequally convex dorsal 
valves : the ventral valves are irregular towards the umbo, and the beaks more 
or less distorted; while the area presents much variety in its degree of elevation. 
In fig. 18, we have a ventral valve with .the strise as regular and even as thQse 
from the limestone ; and in the casts, figs. 19 - 23, we have symmetrical forms with 
even strise, and with form of muscular impression undistinguishable from those of 
the Schoharie grit, the only distinctive feature in those from the Chemung group 
being the tenuity of the shell. 
The varieties illustrated on Plate x are common and often abundant in the 
Chemung group, having a considerable vertical range, and in horizontal range are 
almost coextensive with the formation within the limits of the State. They are 
most abundant and present greatest variety of form in Chemung, Allegany and 
Cattaraugus counties. 
Following the comparison of the New-York Chemung species with similar forms 
of the Waverly sandstone* of Ohio (which I have regarded as of the same age). 
• The term “ subcarboniferous” has been applied by some authors to these sandstones. This is a very 
pernicious nomenclature, as the term is equally applicable to all rocks below the Carboniferous period. 
